1. Field of the Invention
The present invention relates to an apparatus for corona discharge treatment of an article. In particular, the invention relates to an apparatus for corona discharge treatment of a surface of an article comprising a conductive composite in order to enhance the surface adhesion of the article.
2. Description of the Related Art
Corona discharge treatment of surfaces of articles made of thermoplastic polymers is a well-known technique to enhance the surface adhesion of such articles. A number of applications and modifications to corona discharge technology for treating fiber composites based upon thermoplastic polymeric matrices are described in the literature. See, for example, Kodokian et al., "Surface pretreatment and adhesion of thermoplastic fibre-composites", Journal of Materials Science Letters 7 (1988) 625-627, which discloses treating thermoplastic-composite substrates placed on an automatically controlled table which travels horizontally backwards and forwards under a high-frequency voltage, discharge electrode.
Corona discharge devices for treating sheets of material generally comprise a pair of electrodes, at least one of which rotates. For example, U.S. Pat. No. 3,973,132 to Prinz discloses a corona discharge apparatus for treating non-conductive foils comprising a rotating electrode pair, the high-frequency voltage electrode having a profiled cross section, and the ground electrode being a smooth cylinder. When high voltage is applied to one of the electrodes, a corona discharge takes place through the air gap between the electrodes and onto the surfaces of the foil. U.S. Pat. No. 4,273,635 to Beraud et al. discloses a process for corona treatment of bulky fibrous webs derived at least partially from thermoplastic fibers to impart cohesion to the webs. The process includes passing the webs between a pair of rotating cylindrical metallic electrodes. U.S. Pat. No. 4,392,178 to Radice discloses an apparatus for enhancing the piezoelectric properties of polymeric films by corona treatment using a roller electrode mounted for movement along the circumference of a motorized, rotating drum which propels the film. The roller electrode moves in an oscillatory motion normal to the axis of rotation of the drum. U.S. Pat. No. 3,435,190 to Schirmer discloses a corona discharge apparatus used to perforate films of dielectric material. The apparatus includes a stationary, blade-like electrode covered with electrically insulating materials along its length and another elongated, rotatable electrode, between which a sheet of the dielectric material passes. Canadian Patent No. 790,038 to Adams discloses an apparatus for corona treatment of plastic films which conveys a sheet of such plastic film on a cylindrical, rotating roller which acts as one electrode, and passes close to a similarly contoured stationary electrode covered with a layer of dielectric material. The apparatus also has a locknut for adjusting electrode spacing.
In order to obtain uniform corona treatment of an article, surface sparking must be evenly distributed, or the voltage applied by the electrodes will be concentrated in one area and fail to uniformly treat the surface. Achieving uniform corona discharge across the high-frequency voltage electrode requires exacting spacing tolerances, not only between the two electrodes, but in the article-electrode gap as well. Through continued use, gap tolerances are often exceeded, and the discharge uniformity along the high-frequency voltage electrode is destroyed, as evidenced by sparking and pitting of the high-frequency voltage electrode surface. Moreover, most corona discharge devices are not able to maintain even gap spacing when there are thickness variations in the article.
A problem especially arises in the corona treatment of composite articles comprising conductive fibers. In this situation, conventional devices and methods will not be effective, due to increased sparking caused by loose or exposed conductive fibers in the composite. To treat conductive composite articles, high power must be supplied to the high-frequency voltage electrode, since the voltage will seek the conductive fibers in the composite article. This lessens the efficiency of the corona treatment of the surface of the article. Since high power must be supplied, overheating of the electrode, as well as the articles to be treated, becomes a serious obstacle. If the problem is not addressed, the surface of the article may be destroyed by overheating and ablating before the desired surface corona treatment is achieved. Alternately, the increased incidence of sparking can render the corona treatment ineffective. Additionally, at high power levels, any non-uniformity in the electrode is magnified, which further increases the chances that sparking will occur.